A VERY interesting thread, and I'm only responding because I find this fascinating. I'm afraid I don't know any answers for you, but it sounds as though you've done your homework and know more than most here. There may be some guys here who know how to do this. Probably. But if it were me I would also try asking this question over on the Classic Rendezvous list, where there are plenty of very knowledgable fellows who are well versed in old technologies and who are themselves as old as the varnish fix technique.
Just one suggestion, based on absolutely no direct experience; I think I would use artists varnish. A small bottle of Windsor & Newton from the art supply store should be enough and is more of a pure varnish, made from natural ingredients, as opposed to synthetics.
Good luck with your project and please show us the results.

I would use this;
http://www.winsornewton.com/products...pan-gold-size/

and finish the decal with this;
http://www.winsornewton.com/products...gloss-varnish/